1. Field of the Invention
This invention relates to electronic circuitry, and more particularly to active load circuits for operational amplifiers and the like.
2. Description of the Prior Art
Active load circuits have done much to improve the performance and simplify the design of modern linear circuits, especially operational amplifiers and also comparators. An active load, which is essentially a current source substituted for or supplementing a common resistor load, has several distinct advantages. First, it permits low current operation without large resistance values. This is important both in raising impedance levels and in reducing power consumption. Second, it operates over a much wider range of voltages, giving high gain with small voltage drops as well as large linear voltage swings. Also, it makes possible much higher gain per stage, so fewer stages can be used.
Although offering advantages over simple resistor loads, there are still some significant limitations associated with active load circuits when used in the input stage of an operational amplifier and other circuits. For example, an ideal operational amplifier can be considered to be a pure differential amplifier which is insensitive to a common input signal at both input terminals, the situation when the same signal is applied to both differential inputs being referred to as the "common mode". Ideally, there should be no output signal when a common mode input signal is applied. The common mode rejection ratio (CMRR) is a measure of how closely the amplifier approaches the ideal, a CMRR of 100% corresponding to ideal operation. Any variations from a total non-response to a common mode input is reflected in a lower CMRR. Such non-ideal characteristics are a measure of the quality of the components from which the amplifier is manufactured, and the intrinsic balance of the circuit. It would be desirable to increase the CMRR of present active load operational amplifiers.
Present operational amplifiers employing active loads also exhibit a limited common mode voltage range (CMVR), which is the range of input voltages over which the required performance specifications of the amplifier are satisfied. Another problem is that present circuits are also subject to being saturated or cut off during maximum slew rates ("slew rate" being defined as the ratio of change in output voltage to the minimum time required to effect this change under large-signal drive conditions).